PROJECT SUMMARY Thoracic aortic aneurysm and dissection (TAAD) are a poorly understood group of disorders responsible for significant morbidity and mortality in both sexes and all age groups, but without specific pharmacotherapy. Elucidation of disease mechanisms focus on conspicuous areas of medial smooth muscle cell (SMC) loss, whereas foci of SMC proliferation are overlooked. We found that the number of SMCs is increased in clinical specimens of TAAD, as have several other investigators. We considered that excessive SMC proliferation may exacerbate TAAD as dividing cells transition from a contractile phenotype to enter the cell cycle and daughter cells may interrupt interactions with contiguous elastic laminae or drive growth of the vessel wall. To test our hypothesis that SMC proliferation and proliferative signaling contributes to aortopathy, we developed a novel experimental model. Conditional deletion of Tsc1, a component of the tuberous sclerosis complex, in postnatal murine SMCs leads to activation of a key kinase, mechanistic target of rapamycin (mTOR), that regulates cell proliferation among other processes. Our preliminary studies reveal that induction of mTOR signaling and SMC proliferation cause progressive TAAD associated with a novel degradative phenotype of SMCs. Our goals are to understand cellular and molecular mechanisms of the disease process and to determine if relevant in other experimental models and clinical specimens of TAAD. We do not believe that the acquisition of a subset of macrophage markers and functions by degradative SMCs in the aortic media represents transdifferentiation to macrophages as recently described in atherosclerotic plaques. Rather, degradative SMCs acquire certain properties that mimic macrophage maturation, including increased protease secretion, phagocytosis, endocytosis, autophagy, and lysosome activity. Greater proteolysis, together with sequelae from loss of contractile and synthetic activity, lead to elastic fiber fragmentation and TAAD, though clearance of extracellular debris and recycling of macromolecules may retard disease progression. Our hypothesis is provocative and our preliminary data compelling. Completion of our proposed experiments will yield considerable insight into the pathogenesis of TAAD and other mTOR-dependent arteriopathies, such as atherosclerosis and aortic stiffening, and discover new therapeutic targets for this lethal disease.